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   难度：Easy
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   <h1 class="question_title">
    104. Maximum Depth of Binary Tree
   </h1>
   <p>
    Given a binary tree, find its maximum depth.
   </p>
   <p>
    The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
   </p>
   <p>
    <strong>
     Note:
    </strong>
    &nbsp;A leaf is a node with no children.
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <p>
    Given binary tree
    <code>
     [3,9,20,null,null,15,7]
    </code>
    ,
   </p>
   <pre>
    3
   / \
  9  20
    /  \
   15   7</pre>
   <p>
    return its depth = 3.
   </p>
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   <h1 class="question_title">
    104. 二叉树的最大深度
   </h1>
   <p>
    给定一个二叉树，找出其最大深度。
   </p>
   <p>
    二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。
   </p>
   <p>
    <strong>
     说明:
    </strong>
    &nbsp;叶子节点是指没有子节点的节点。
   </p>
   <p>
    <strong>
     示例：
    </strong>
    <br>
    给定二叉树
    <code>
     [3,9,20,null,null,15,7]
    </code>
    ，
   </p>
   <pre>    3
   / \
  9  20
    /  \
   15   7</pre>
   <p>
    返回它的最大深度&nbsp;3 。
   </p>
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